Implement esp_yield() as a replacement for delay(0)

esp_yield() now also calls esp_schedule(), original esp_yield() function renamed to esp_suspend().

Don't use delay(0) in the Core internals, libraries and examples. Use yield() when the code is
supposed to be called from CONT, use esp_yield() when the code can be called from either CONT or SYS.
Clean-up esp_yield() and esp_schedule() declarations across the code and use coredecls.h instead.

Implement helper functions for libraries that were previously using esp_yield(), esp_schedule() and
esp_delay() directly to wait for certain SYS context tasks to complete. Correctly use esp_delay()
for timeouts, make sure scheduled functions have a chance to run (e.g. LwIP_Ethernet uses recurrent)

Related issues:
- #6107 - discussion about the esp_yield() and esp_delay() usage in ClientContext
- #6212 - discussion about replacing delay() with a blocking loop
- #6680 - pull request introducing LwIP-based Ethernet
- #7146 - discussion that originated UART code changes
- #7969 - proposal to remove delay(0) from the example code
- #8291 - discussion related to the run_scheduled_recurrent_functions() usage in LwIP Ethernet
- #8317 - yieldUntil() implementation, similar to the esp_delay() overload with a timeout and a 0 interval

tests/host/common/Arduino.cpp

@@ -15,6 +15,7 @@
 
 #include <sys/time.h>
 #include "Arduino.h"
+#include <functional>
 
 #include <unistd.h>
 
@@ -53,10 +54,31 @@ extern "C" void optimistic_yield (uint32_t interval_us)
     (void)interval_us;
 }
 
+extern "C" void esp_suspend()
+{
+}
+
+extern "C" void esp_schedule()
+{
+}
+
 extern "C" void esp_yield()
 {
 }
 
+extern "C" void esp_delay (unsigned long ms)
+{
+    usleep(ms * 1000);
+}
+
+bool esp_try_delay(const uint32_t start_ms, const uint32_t timeout_ms, const uint32_t intvl_ms) {
+    uint32_t expired = millis() - start_ms;
+    if (expired >= timeout_ms) {
+        return true;
+    }
+    esp_delay(std::min((timeout_ms - expired), intvl_ms));
+    return false;
+}
 
 extern "C" void __panic_func(const char* file, int line, const char* func) {
     (void)file;
@@ -67,7 +89,7 @@ extern "C" void __panic_func(const char* file, int line, const char* func) {
 
 extern "C" void delay(unsigned long ms)
 {
-    usleep(ms * 1000);
+    esp_delay(ms);
 }
 
 extern "C" void delayMicroseconds(unsigned int us)
@@ -77,6 +99,6 @@ extern "C" void delayMicroseconds(unsigned int us)
 
 #include "cont.h"
 cont_t* g_pcont = NULL;
-extern "C" void cont_yield(cont_t*)
+extern "C" void cont_suspend(cont_t*)
 {
 }

tests/host/common/include/ClientContext.h

@@ -24,9 +24,6 @@
 class ClientContext;
 class WiFiClient;
 
-extern "C" void esp_yield();
-extern "C" void esp_schedule();
-
 #include <assert.h>
 
 bool getDefaultPrivateGlobalSyncValue ();

tests/host/common/user_interface.cpp

@@ -498,10 +498,6 @@ extern "C"
         (void)intr;
     }
 
-    void esp_schedule(void)
-    {
-    }
-
     void dns_setserver(u8_t numdns, ip_addr_t *dnsserver)
     {
         (void)numdns;